Nobody actually took the
time to explain
exactly what it is...
Systematic downvoting is just voting down large numbers of writeups due to who the author is, without regard for the content. Often done by going to the user search and just looking at one writeup after another, voting on each in sequence, though sometimes it is more random, and may just be a grudge held by a noder causing them to downvote each writeup they run across without hunting for them.